Apart from intensity-related evaluation methods, conventional high-precision optical position measuring devices also use polarization-optical methods for the generation of three or more phase-displaced, displacement-dependent scanning signals in order to ascertain a position. As far as the polarization-optical generation of scanning signals is concerned, reference is made to European Published Patent Application No. 0 481 356, for example.
The principle on which the signal generation according to European Published Patent Application No. 0 481 356 is based is schematically illustrated in FIG. 1, in a schematic illustration of the unfolded scanning beam path. Gratings A, M are jointly disposed along the indicated measuring direction x in a manner that allows them to move in relation to the other components L1, L2, AO. The beam bundle impinging from the left, which is polarized in a defined manner by polarizer P1, is split into two partial beam bundles via grating A. Polarization-optical elements, such as differently oriented lambda quarter-wave plates PE1, PE2, for example, are provided in the beam path of the partial beam bundles to be brought into interference, in which the positional information is encoded. The lambda quarter-wave plates PE1, PE2 polarize the two partial beam bundles passing through in a mutually orthogonal manner, i.e., the two partial beam bundles are then polarized in a left-hand-circular and right-hand-circular manner, for example. These two partial beam bundles are subsequently superposed to form a common signal beam bundle (0) and split up into three or more superposed partial beam bundles 190, 1210, 1330 in a downstream evaluator optics system AO. After passing through polarizers P90, P210, P330 of different orientations, scanning signals S90, S210, S330, each phase-displaced by 120° and able to be processed further in, e.g., a conventional manner, ultimately result at detector elements D90, D210, D330. In addition to lambda quarter-wave plates PE1, PE2, still further polarization-optical components in the form of polarizers P2, P3 are frequently placed in the beam paths of the split-up partial beam bundles in order to compensate for faulty polarizations, which were caused by gratings A, L1, L2 through which the partial beam paths passed earlier.
One disadvantage of such a polarization-optical generation of multiple phase-displaced and displacement-dependent scanning signals is the necessity of placing additional optical elements, such as lambda quarter-wave plates and polarizers, in the scanning beam path or in the scanning gap between the components that are moved relative to each other. Given limited space of the corresponding optical position measuring device or a short provided scanning distance, such additional components may pose a problem. If the position measuring device is to have a design similar to the principles described in PCT International Published Patent Application No. WO 2008/138501, components A, M and L1, L2 illustrated in FIG. 1 are provided in the form of two measuring standards, which are displaceable relative to each other. In this case, mechanically fixing interposed stationary polarization-optical components is frequently not possible.
Moreover, the additional polarization-optical components in the scanning beam path also pose greater demands on the evenness, parallelism, and homogeneity of the employed support structures. Required are drift-free and stable assembly surfaces for accommodating these support structures. In such a case, possible material faults are able to be compensated for only at very high additional expense with the aid of appropriate calibration methods. This is true, in particular, if long, translation-invariant scales are used in the corresponding optical position measuring devices.
Moreover, additional system properties such as natural frequencies and air flows in the scanning gap may also be negatively affected by additionally required polarization-optical components within the scanning beam paths.
An optical position measuring device featuring a polarization-optical generation of phase-displaced scanning signals without additional discrete polarization-optical components in the scanning beam paths is described, for example, in European Published Patent Application No. 2 466 272. According to this solution, the required polarization-optical components are designed to be integrated into other components of the scanning beam path, e.g., in the form of high-frequency gratings having periodically varying structures. The components used in such a position measuring device exhibit position-dependent polarization characteristics in the measuring direction. For example, the measuring standard is made up of multiple locally variable layers in this case and includes a high frequency grating having a graduation period dR<λ/2, which has only a 0th order of diffraction and produces the polarization-optical functionality. The grating orientation of the high-frequency grating varies by the polarization period dp>hwspot along the measuring direction, which must be considerably greater than width hwspot of the illuminated region of the measuring standard delimiting the signal period SP of the generatable scanning signals in the downward direction. Since the 0th order of diffraction of the high frequency grating is unable to induce a geometric beam deflection, another periodicity dT of a geometrically deflecting grating is required in addition, which is large enough to generate at least one first order of diffraction, which means that dT>λ/2 must apply. Because polarization periodicity dP is not intended to lead to a geometric deflection either, it is selected considerably greater than periodicity dT.
The following condition must therefore apply with regard to the dimensioning of the three periodicities dR, dT, and dP that arise in the different gratings:dR<λ/2<dT<hwspot<dP<4 SP.
As a matter of principle, gratings having small periodicities dR can be produced only by a technologically complex process. On the other hand, a small signal period SP is desirable for high resolution of the corresponding position measuring device. As a result, the position measuring device described in European Published Patent Application No. 2 466 272 is subject to certain restrictions with regard to the small available dimensioning range between dR and SP that may be utilized for the periodicities dT and dP of the corresponding gratings. Furthermore, certain restrictions exist with regard to width hwspot of the illuminated region of the measuring standard.